The present invention relates to a loudspeaker, and more particularly to a diaphragm provided in the loudspeaker and the structure of members surrounding the diaphragm in a loudspeaker.
A loudspeaker provided in an audio system is an electroacoustic device that converts an electric signal (electrical energy) into an acoustic signal (sound energy). On the basis of operational principles, the loudspeakers are roughly divided into electrodynamic, electrostatic, piezoelectric, discharge, and electromagnetic speakers. Electrodynamic loudspeakers, which are superior in reproduced frequency range and converting efficiency are widely used today.
One of the electrodynamic loudspeaker is a cone loudspeaker. FIG. 6 shows an example of a conventional cone loudspeaker. Referring to the figure, the conventional cone loudspeaker has a pole yoke 106, magnet 104 mounted on the yoke 106, and a plate 105 mounted on the magnet, thereby forming a magnetic circuit 107 including an annular magnetic gap. A frame 112 is attached to the plate 105.
A conical diaphragm 101 having a center hole is mounted above the pole yoke 106. The outer periphery of the diaphragm 101 is secured to the frame 112 through an edge 108 having appropriate compliance and rigidity.
A cylindrical voice coil bobbin 103 is provided in the hole of the diaphragm 101, the upper periphery attached thereto. A voice coil 102 is mounted in a space surrounding the bobbin 103.
The voice coil bobbin 103 is supported by the frame 112 through an annular damper 109 having an appropriate compliance and stiffness. The outer periphery of the damper 109 and the edge 108 are secured to the frame 112 integral with the magnetic circuit 107 so as to resiliently support the diaphragm 101 and the voice coil bobbin 103 at the respective predetermined static positions within the magnetic gap without contacting the members of the magnetic circuit 107.
The ends of the voice coil 102 are connected to conductive leads 111. Each lead 111 is connected to a terminal 110 provided on the frame 112.
In order to reinforce the structural strengths of the diaphragm 101 and the voice coil 102, a center cap 113 is mounted on the center portion of the diaphragm 101, so as to cover the center hole thereof. Hence partial vibrations of the diaphragm 101 and the voice coil 102 are prevented.
In the thus constructed loudspeaker, when applied with audio current through the terminals 110 and the leads 111, the voice coil 102 is electromagnetically driven in the magnetic gap of the magnetic circuit in the vibrating direction of the diaphragm 101. Thus, the diaphragm 101 is vibrated in the axial direction, thereby generating sounds.
Hence, it is possible for the diaphragm 101 to be deflected toward the front in the vibrating direction until the edge 108 is fully tensed. In the rearward direction, the diaphragm 101 can be deflected until the inner periphery of the damper 109 attached to the voice coil bobbin 103 abuts against the plate 105. Such a range is the maximum vibrating quantity of the loudspeaker.
Since the center cap 113 integrally vibrates with the diaphragm 101, the center cap 113 serves to radiate a part of the acoustic energy, mainly the sounds in the high frequency range. In addition, the center cap 113 compensates the deflection in the phase of the sound waves caused by interference which is generated due to the shape of the diaphragm 101, thereby adjusting the acoustic characteristics of the loudspeaker, and further corrects as necessary, the influence of the center hole of the diaphragm 101 on the acoustic characteristics.
The loudspeaker is recently used not only as an independent device composing an audio system, but also disposed in a door of a motor vehicle, a casing of a flat electronic display, and in other small spaces within casings of various shapes. In such a instance, it is necessary to manufacture a thin loudspeaker which is reduced in height as much as possible so that the loudspeaker may be easily mounted in a casing of a limited size.
In order to reduce the thickness of the conventional corn loudspeaker, the height of the diaphragm, which takes up most of the height of the loudspeaker, must be reduced. The height of the speaker can be reduced by increasing the half vertical angle .theta. formed between the diaphragm and the voice coil bobbin as shown in FIG. 6, provided the caliber, that is the outer circumferential diameter of the diaphragm is the same.
In the cone loudspeaker, as the half vertical angle .theta. decreases, the rigidity of the diaphragm in the vibrating direction generally increases, thereby restraining the partial vibration of the diaphragm. As a result, the reproduction frequency range of the loudspeaker can be expanded to the higher frequency range. However, the partial vibration is liable to occur as the vertical angel increases under the same condition concerning the caliber and the material of the diaphragm. Thus, there occurs disturbances in the reproduced frequency response so that the reproduction frequency range becomes limited, especially in a high frequency range. Therefore, if the reproduction frequency in the high frequency range is to be maintained to a degree, the vertical angle cannot be largely increased.
Moreover, since the diaphragm vibrates in the axial direction when the loudspeaker is in operation, in a case of the speaker mounted in a casing, the actual height of the speaker is determined in consideration to the amplitude of the diaphragm. As is widely known, if the loudspeaker is mounted on an infinite rigid wall for example, the amplitude of the diaphragm is inversely proportional to the square of a frequency in a constant output range wherein the axial vibration occurs, and the amplitude becomes maximum at a frequency approximately equal to the minimum resonance frequency. Furthermore, the amplitude increases in proportion to the driving input of the loudspeaker.
Consequently, in order to set the minimum resonance frequency of the speaker at a low frequency so that the reproduction frequency range is more or less extended in a lower frequency range in a casing having a limited inner space, the maximum amplitude of the diaphragm must be increased. Hence a space which allows the diaphragm to be projected at the maximum amplitude must be provided in the casing wherein the speaker is mounted. The thickness of the speaker must be reduced to provide for such a space.
In addition, in order to allow the maximum vibration of the diaphragm, the distance between the damper 109 and the plate 105 must be so designed as to be proportional to the maximum amplitude. Thus the height of the voice coil bobbin 103 is increased, thereby increasing the height of the speaker.
Moreover, when increasing the allowable input of the loudspeaker, since a large amplitude is necessary, the size of a mounting portion in the casing, and the height of voice coil bobbin 103 must be increased as described above. When the height of the voice coil bobbin 103 is increased, the vertical position of the voice coil 102 which is electromagnetically driven is deviated from the vertical position of the inner periphery of the damper 109, thereby destabilizing the resilient support of the voice coil 102, voice coil bobbin 103 and the diaphragm 101 in the vibrating direction. As a result, a so-called rolling of the voice coil and the voice coil bobbin frequently occurs when the loudspeaker is driven.
When the rolling occurs, voice coil 102 and voice coil bobbin 103 violently crash and rub against the plate 105 and the pole yoke 106 in the magnetic gap, so that an allophone is generated from the diaphragm 101, and in extreme cases, the voice coil 102 is cut. Hence the amplitude cannot be increased although the height of the voice coil bobbin 103 is increased. Consequently, the allowable input of the loudspeaker cannot be increased.
Thus, there is a limit in decreasing the thickness of a conventional loudspeaker wherein the wide reproduction frequency range and allowable input are necessary.